This invention relates to security alarm systems such as disclosed in our U.S. Pat. No. 3,939,460 issued on Feb. 17, 1976, for a "Security Alarm System".
The above noted copending patent application discloses a security alarm system in which a plurality of burgular and fire alarm conductors from different premises are wired to a multiplexer in a central data transmitter unit. A binary counter is coupled to the multiplexer and a clock pulse circuit is coupled to the binary counter to step the counter through its counting cycle. The counter causes the multiplexer to connect the alarm conductors one at a time in sequence to the input of an alarm detector which detects alarm conditions on the conductors. When an alarm condition is detected, the counter is stopped and the number in the counter at that time is displayed on a visual display in the data transmitter along with a letter signifying whether the alarm is a fire alarm or a burgular alarm. The displayed number, which indicates the premises in which the alarm condition is detected, is also transmitted by the data transmitter to a remote location, e.g. to a police station or fire station. After the alarm condition and number have been displayed and transmitted, the counter is re-activated and resumes stepping the multiplexer through its scanning cycle.
One problem associated with the above described security alarm system is that an alarm condition for any particular alarm conductor is re-transmitted every time the multiplexer scans the corresponding alarm conductor as long as the alarm remains active. For example, if a fire alarm is detected at a certain premises, the fire alarm and number of the premises would be repeatedly transmitted to the fire department on every multiplexer scan until the fire is put out, and perhaps longer, since the fire may short or open the fire alarm conductor and possibly cause the alarm condition to remain until somebody repairs the fire alarm conductor. Such continual re-transmissions of the alarm are unnecessary and undesirable. Accordingly, it is desirable to limit the number of times that any particular alarm is transmitted to the remote location.
Another problem in the prior art security alarm systems is that the person assigned to monitor the data alarm transmitter may not spend all of his or her time at the transmitter and thus might miss one or more of the alarms. For example, the data alarm transmitter may be in an apartment house and the security man for the apartment house may be responsible for monitoring the alarm display and taking the appropriate action. However, such a security man would normally have other duties and might miss an alarm while away from the data transmitter unit. As a result, a small fire which could be easily put out if attended to promptly might engulf the building before the fire fighters arrived. Accordingly, it is desirable to provide an alarm memory in the data transmitter unit with which an operator can recall previous alarms that were registered in his absence.